Ndileka Mandela, 52, a nurse who runs a Mandela family foundation to uplift the rural poor, said she had been left despondent by the ANC-led government, which she said was squandering public money and neglecting the poor.

“This is not a decision that has been made out of anger,” said Ms Mandela, who is the first in the family to reject the ANC.

“I've been thinking about it for a while. It's been a build-up.”

She said worthwhile projects in rural areas were always held up by a lack of funding, yet the government, led by President Jacob Zuma, was wasting billions of rand.

The ANC’s callousness after a government blunder caused the deaths of 96 state psychiatric patients in the Gauteng province last year was the tipping point in her decision, she added.

Nelson Mandela embracing Fidel Castro

The patients were part of a group of 1,300 who were transferred from a private hospital to 27 charitable organisations as a way of saving money.

Many of the bodies of the patients were found to have head injuries and unexplained bruises. At the opening of parliament in February, the ANC-elected speaker refused a request from MPs to hold a moment of silence of the patients.

“It's one scandal after the next and there's no accountability. And our people suffer for it.”

Ms Mandela, whose father died in a car accident in 1969, said her grandfather would never have supported blind loyalty.

She does not know who she will vote for yet, but said she is looking for a party that will uplift rural areas.

“I will not be voting for something that does not resonate with me anymore, and does not resonate for what granddad and his comrades fought for,” she said.

First known TV interview with Nelson Mandela

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The ANC has been beset with corruption scandals since Mr Zuma, 74, was elected to office in 2009. Last year the country’s highest court found that he had violated his oath of office by refusing to abide by a report by a state watchdog to pay back public money spent on upgrading his rural home.

Mr Zuma paid back 7.8-million rand (500,000 pounds) for the upgrades, which included a swimming pool and state of the art chicken coup, in September last year

He also faces the reinstatement of 783 corruption charges linked to a multi-billion pound arms deal nearly 20 years ago.

The party, which has won every election with more than 60 per cent of the vote since 1994, suffered its worst ever election result in August last year, losing the key municipalities of Johannesburg, Pretoria and Port Elizabeth to the opposition Democratic Alliance party.

Mr Mandela, who died in December 2013, was elected South Africa’s first black president in 1994 after spending decades in prison for fighting against white minority rule.